an engine to ground should be somewhere near the battery or in the trunk..
this is for an amplifier, if so.. you could sand a spot in the trunk and use that, shorter the ground, the better.. should also use the same gauge wire as your amplifiers power cable.. keep the fuse within a foot or three of the battery.
mount your amplifer as per the manual, for heat disipation
what im trying to do is called the big three. You upgrade your battery to chassis ground, the alternator to battery power, and engine to chassis ground. is supposed to help with light dimming with a powerful audio system. Ive done the first two I just cant find the engine to chassis ground.